Exchange bias study of CoFeB/IrMn antidot and nanodot arrays fabricated by nanosphere lithography

Abstract

Exchange-biased bilayers are widely used as reference layers in nanometric spintronic devices. While the magnetic properties of polycrystalline multilayers are widely reported, the exchange bias effect in nanostructures containing amorphous ferromagnetic layers are not systematically investigated. In this work, CoFeB/IrMn antidot and nanodot arrays were patterned by nanosphere lithography. The average diameter of pores in the antidot arrays changes from 88 nm to 105 nm as the thin film thickness increases from 20.8 nm to 28.5 nm, while the size of nanodots are kept at 60 nm. The exchange bias and coercivity of the nanostructures and continuous films exhibit similar exponential dependence on CoFeB layer thickness. The vacuum annealing under magnetic field results in structural relaxation, interdiffusion, and CoFeB crystallization, as the annealing temperature is increased from 373 K to 673 K. These mechanisms are responsible for the changes in crystallinity, surface roughness, and magnetic properties. Reduced coercivity and enhanced exchange bias is observed after annealing at low temperatures, while high-temperature annealing (> 573 K) results in higher coercivity and lower exchange bias.This work provides physical insights on the magnetization reversal response in nanosized spintronic devices involving CoFeB/IrMn reference layers.